1. Field of the Invention
This invention is directed to a thermistor for temperature measurement, control and/or temperature compensation and a method of making the same. More particularly, the invention is directed to a thermistor having multiple layers of electrode metal.
2. Background of the Invention
Thermistors (thermally sensitive resistors), are ceramic semiconductors which exhibit large changes in electrical resistance with corresponding changes in temperature. Because of their sensitivity, accuracy, and stability, thermistors are generally accepted to be the most advantageous sensor for many applications including temperature measurement, compensation, and control. Thermistors are used extensively for commercial consumer electronic products, automotive, industrial, and medical electronic applications, food handling and processing, communication and instrumentation, computers, military and aerospace, and research and development. Some practical uses of thermistors include liquid level measurement, photography, thermometers, intravenous catheters, blood analysis, myocardial needle probes, automotive climate control, fuel level/temperature, temperature sensors for household appliances such as air conditioners, coffee makers, and data logger applications such as air, soil, liquid temperature probes. Their use in portable phones, carphones, quartz oscillators and transceivers continues to expand.
The most important characteristics of thermistors are an extremely high temperature coefficient of resistance and precise resistance versus temperature characteristics. The sensitivity to temperature change can result in a thermistor resistance change of 10 million to one over an operating temperature range. Prior art chip thermistors are of small size, square configuration, are available coated or uncoated, in leaded or unleaded embodiments, having operating temperature ranges of xe2x88x9280xc2x0 C. to 300xc2x0 C., and resistance ranges from 0.5 ohms to 40 megohms.
The electrical resistivity of a positive temperature coefficient (PTC) thermistor increases with increase in temperature. PTC thermistors switch from a low resistance to a high resistance state at a specific temperature. They are widely used as current limiters from xe2x88x9280xc2x0 C. to 300xc2x0 C., 0.5 ohms to 40 megohms. Conversely, the electrical resistivity of a negative temperature coefficient (NTC) thermistor decreases with increase in temperature. NTC thermistors are used to sense temperatures from xe2x88x9280xc2x0 C. to 300xc2x0 C. with nominal resistance at 25xc2x0 C. from 0.5 ohms to 40 megohms. Therefore, they have a large temperature coefficient of resistance and a wide range of resistance values. They are also available in a wide range of sizes from 3 mm in diameter to 22 mm in diameter in adaptable shapes and sizes for a wide variety of mechanical environments. Typical applications for NTC thermistors include fan control, temperature sensing, circuit protection and temperature control. NTC thermistors are chosen when remote sensing is required, small size is desired, or where small temperature differences need to be measured.
NTC thermistors used for temperature measurement and compensation are usually made from various compositions including the oxides of manganese, nickel, cobalt, copper, iron, and other metals to form a ceramic semiconductor material. Thermistors may be formed into different shapes of bead, disc, rod, chip or flake configuration. The flake style thermistor is simply a much smaller size version of the chip thermistor. Wafer thermistors are produced by forming thin sheets of material including powders of the oxides of manganese, nickel, and other oxides combined in a binder. The material is sintered at elevated temperatures, coated with a conductive metal composition, and then diced to size. Leads are attached by soldering. The units are finally coated in an epoxy or other electrical insulation material for final protection and stabilization. A typical prior art thermistor element, shown in FIG. 1, represents a chip type thermistor composed of sintered powders of metal oxides (1) on which electrodes (2) and (3) are deposited.
Specifically, when the prior art thermistors with thick film electrodes made with Ag, PdAg or Au are attached to substrates (surface mount configurations, FIG. 1) or to leads (discrete component configurations, FIG. 2) with high temperature solders using processes operating between 200xc2x0 C. and 380xc2x0 C. at dwell times ranging from 5 seconds to 3 minutes, their electrical resistance shifts outside the allowable specified resistance tolerance (typically 2-5%). This results in a defective or deficient final product or sub-assembly into which the thermistor is assembled.
These resistance shifts of the prior art thermistors have now been found to be caused by a phenomenon called leaching, which occurs during the soldering process. Leaching occurs because the metal in the electrode has a higher affinity for the molten solder than its bond with glass frit or fritless binder of the electrode. As the thermistor electrode is being soldered, the metal is released from its bond with the glass frit or fritless binder of the electrode and is absorbed into the molten solder. As a result, the electrical resistance of the thermistor increases from its original value, prior to the soldering process. In other words, the metal element forming the external electrodes will be compromised due to the solder leaching.
The rate of leaching of the thermistor thick film electrode is dependent on the type of electrode material and the temperature and the duration of the soldering process to which the thermistor is exposed. Typically, exposing thermistors of the prior art to molten solder at temperatures above 200xc2x0 C. for extended periods of time (greater than 5 seconds) is not recommended by thick film electrode manufacturers since degradation of the electrode increases more rapidly above this temperature and beyond this time. In addition to the shift in electrical resistance, leaching causes degradation of the solder-electrode and electrode-semiconductor bond. Weakened bonds may result in thermistors having greatly reduced stability and reliability.
Thick film Pt electrodes have been found to be resistant to leaching compared to other electrode materials. However, the high cost of thick film Pt electrodes renders the prior art thermistor not cost effective to manufacture. Also, it is more difficult to bond Au wire to thick film Pt electrodes using the thermo-sonic or equivalent wire ball bonding process.
In addition, prior art thermistors with thick film Ag or PdAg electrodes are not commonly used in hybrid microcircuit applications requiring 0.001xe2x80x3 OD gold wire (or equivalent) to be bonded using the thermo-sonic or equivalent wire ball bonding process because the wire bonds to these electrodes may not be reliable over the long term.
A thermistor element using two layers of thin film electrodes have been described in the prior art (U.S. Pat. No. 4,712,085). Other prior art (U.S. Pat. No. 6,008,717) describes a thermistor with a pair of electrodes in a shorter inner electrode and a longer inner electrode are mutually opposite each other and separated by a gap. However, this prior art does not solve the leaching problem described.
Based on the above, it is an object of the present invention to provide a a cost effective thermistor with electrodes having a high degree of solder leach resistance and able to withstand soldering processes operating at temperatures typically between 200xc2x0 C. and 380xc2x0 C. with dwell times typically between 5 seconds and 3 minutes.
Another object of the present invention is to provide for the manufacture of a thermistor with leach resistant electrodes that allow for use of higher temperature solders or low fire conductive compositions to attach leads, thereby increasing the maximum operating temperature capability over that achieved in the prior art.
These and other objects are achieved by the present invention directed to a thermistor having a semiconductor body with a first electrode layer deposited outward from opposite surfaces of the semiconductor body. The first layer, having a thickness of not less than about 5 micrometers, is formed from an electrode material which may be any suitable conductive metal. The second layer is deposited outward from the first layer and has a thickness of not more than about 5 micrometers. The second layer is formed from an electrode material which may be any xe2x80x9creactive metalxe2x80x9d. The third electrode layer is deposited outward of said second layer and has a thickness of preferably not more than about 5 micrometers. The third electrode layer is formed from electrode material which may be any xe2x80x9cbarrierxe2x80x9d metal. The fourth layer, which is optional depending on the electrical contacts to be bonded thereto, is formed from an electrode material compatible with the electrical contact and/or means for bonding thereto, outward of the third layer and preferably have a thickness of not more than about 5 micrometers. Each of the layers are in electrical contact with the other layers and the semiconductor body.
For the purposes of this disclosure, metals are considered to be any metal, combination of metals or metal alloys. xe2x80x9cReactivexe2x80x9d metals are considered to be metals, including any combination or alloy, that react at some level with an adjacent metal to provide improved bonding. xe2x80x9cBarrierxe2x80x9d metals are considered to be metals, including any combination or alloy, that resists leaching, i.e. migration of the metal into the solder under high temperature conditions, making them suitable for high temperature soldering processes thereby preventing degradation of the layers beneath.
The present invention can be used with any type semiconductors derived from any suitable processes known in the art including but not limited to disc, rod, chip and flake semiconductors. The present invention applies to PTC or NTC semiconductors.
The method for manufacture of the thermistors of this invention include applying the first layer to the semiconductor body by any known means. The subsequent layers are then deposited outward of the first layer so that the reactive layer is outward of the first layer and the barrier layer is outward of the reactive layer. If the electrical contacts can be bonded to the barrier layer no additional layer is contemplated. However, if the contact is not compatible with the barrier metal, an optional fourth layer is applied over the barrier layer. The choice of metals depends on the type of die and/or wire bonding materials to be used for attaching the thermistor.
As a result of the leach resistant properties imparted by the present invention, the thermistor demonstrates much greater stability and reliability both during and after the soldering process used for attaching said thermistor to substrates than that achieved with prior art. For example, a thermistor element of the present invention soldered to an electrical contact under the same conditions and using the same die and/or wire bonding techniques and processes as previously described for the prior art showed resistance shifts of less than 1% as compared to 6% to 20% for a thermistor of the prior art with thick film Au electrodes.